1. Field of the Invention
This invention relates to hybrid fiber constructions having reduced creep tendency. More particularly, the invention pertains to a twisted, low creep yarn formed by twisting together one or more high strength polyolefin fibers and one or more low creep reinforcing fibers.
2. Description of the Related Art
It is preferable to use light weight, high strength fibrous reinforcements in composite applications for use in demanding environments such as sporting goods, aircraft parts, conveyor belts and for the formation of high pressure tubular structures such as pipes, hoses and other conduits. High performance thermoplastic fibers, such as polyolefin fibers, are excellent materials to form these composite structures because they have very high strength to weight performance. For example, U.S. Pat. No. 4,608,220 teaches fiber reinforced fibrous composites used for the manufacture of aircraft parts. U.S. Pat. No. 6,804,942, for example, teaches composite tubular assemblies formed from polymeric tubes that are wrapped with reinforcing fabric strips. Such high pressure tubular structures are designed to operate under extreme conditions, where they must withstand chemical and mechanical effects caused by their transport of gases and liquids.
High performance thermoplastic fibers are also known to be useful for the formation of articles having excellent ballistic resistance or cut resistance. For example, U.S. Pat. No. 6,979,660 teaches protective fabrics formed from untwisted polyethylene yarns. U.S. Pat. No. 4,886,691 teaches cut resistant articles where a less cut resistant member is surrounded by a more cut resistant jacket material. The cut resistant jacket material may be formed from yarns that include a non-twisted longitudinal polyolefin fiber strand which is wrapped by a second fiber. Accordingly, fibrous composites have been used in a variety of industries for a variety of applications.
While certain polymeric fiber types are known to have certain benefits, they are also known to have certain disadvantages. For example, while polyolefin fibers are known to have excellent strength to weight performance, it has been found that they are more susceptible to long term creep than aramid or carbon fibers. Over time, long term creep effects may result in fiber breakage and compromise the integrity of fibrous articles. In some applications, such as high pressure pipes and hoses, a compromise in the composite integrity can potentially cause significant harm to consumers, surrounding infrastructure and the environment. Nonetheless, the attractive strength to weight properties of polyolefin fibers make them highly desirable materials for such demanding applications. Accordingly, there is a need in the art for high performance composite structures formed with high strength polyolefin fibers but having a reduced creep tendency. The present invention provides a solution to this need.